Phase diagram of Bi₂Sr₂CaCu₂O_8+δ revisited

In cuprate superconductors, the doping of carriers into the parent Mott insulator induces superconductivity and various other phases whose characteristic temperatures are typically plotted versus the doping level p. In most materials, p cannot be determined from the chemical composition, but it is derived from the superconducting transition temperature, T_c, using the assumption that the T_c dependence on doping is universal. Here, we present angle-resolved photoemission studies of Bi₂Sr₂CaCu₂O_8+δ, cleaved and annealed in vacuum or in ozone to reduce or increase the doping from the initial value corresponding to T_c = 91 K. We show that p can be determined from the underlying Fermi surfaces and that in-situ annealing allows mapping of a wide doping regime, covering the superconducting dome and the non-superconducting phase on the overdoped side. Our results show a surprisingly smooth dependence of the inferred Fermi surface with doping. In the highly overdoped regime, the superconducting gap approaches the value of 2Δ₀ = (4 ± 1)k_BT_c.